As discussed in detail in U.S. Pat. No. 4,312,842 and in co-pending applications Ser. No. 342,474 and Ser. No. 551,051 (the substances of which are herein incorporated by reference), this invention relates to the wet process or acid process of making phosphoric acid, and particularly relates to the removal of fluorine and phosphorus values from the sulfur dioxide containing gases evolving from the preheater and the calciner/furnace treatment of the waste phospho-gypsum. The gas stream is cleaned of fluorine to a level which makes the gas acceptable for use with a vanadium oxide catalyst (or other type catalyst) in a contact sulfuric acid plant and the gas has a composition which is desired for efficient operation of the sulfuric acid plant.
The prior art is succinctly described in U.S. Pat. No. 4,312,842 and in co-pending U.S. Pat. No. 4,415,543 and the present application is different from and in certain aspects an improvement on U.S. Pat. No. 4,415,543 in that we eliminate the water wash and filter step of phospho-gypsum treatment just prior to preheating. Instead, we now prefer to secure the phospho-gypsum directly from the final filter or stockpile of the conventional plant operation, pelletize it or otherwise intermix it with coke or other source of carbon, and heat it in a four stage operation.
The first stage is a preheater or dryer where combustion gases heat the phospho-gypsum to over 200 degrees C. After the pellets are preheated, the combustion gases contain water vapor, fluorine and phosphorus and are fed to a condenser which will remove most of the water, fluorine, and phosphorus. These values are then recovered by conventional technology. The gases also contain sulfur dioxide gas from the fossil fuel and the gases are then fed to a clinker cooler where they are passed over a hot clinker and scrubbed by the lime in the clinker.
The second and third stages are a calciner having first a reducing atmosphere zone, followed by a higher temperature oxidizing atmosphere zone. Gases evolved from the calciner are sent to a scrubber system.
The solid residue of the waste phospho-gypsum is then placed in an electric furnace (fourth stage) where the remaining volatiles are evolved. These gases are combined with the gases from the calciner prior to entry in the scrubber. The gas scrubber, which is a major element of this invention, consists of a first sulfuric acid gas scrubber followed by a water scrubber. This unique combination removes virtually all of the fluorine and phosphorus from the gases, leaving them in a form acceptable and desirable for use in a modern contact process sulfuric acid plant.